Since lithium ion or lithium secondary batteries can achieve high energy density, these attract attention as power sources for cell phones and notebook computers, and additionally also as large power sources for electricity storage and power sources for automobiles.
Although lithium ion or lithium secondary batteries can achieve high energy density, up-sizing makes the energy density gigantic, and higher safety is demanded. For example, in large power sources for electricity storage and power sources for automobiles, especially high safety is demanded. Therefore, there are applied safety measures including the structural design of cells, packages and the like, protection circuits, electrode materials, additives having an overcharge protection function, the reinforcement of shutdown function of separators, and the like, thus securing the safety of secondary batteries.
Lithium ion secondary batteries use aprotic solvents such as cyclic carbonates and chain carbonates as an electrolyte solution solvent, and these carbonates tend to have a low flash point and be combustible though having a high dielectric constant and a high ionic conductivity of lithium ions.
A technology is known which uses as an additive a substance which is reductively decomposed at a higher potential than those of carbonates used as electrolyte solution solvents and forms an SEI (Solid Electrolyte Interface) that is a protection membrane having a high lithium ion permeability. The SEI has large effects on the charge/discharge efficiency, the cycle characteristics and the safety. The SEI can reduce the irreversible capacity of carbon materials and oxide materials.
As one of means to further enhance the safety of lithium ion secondary batteries, there is a method in which electrolyte solutions are made to be flame retardant. Patent Literature 1 discloses an organic electrolyte solution secondary battery which uses a phosphate triester as a main solvent of an organic electrolyte solution and in which negative electrode contains a carbon material as a main constituting element. Patent Literature 2 discloses that the use of a phosphate triester as an organic solvent of an electrolyte solution can improve the safety.
Patent Literature 3 discloses a secondary battery in which a nonaqueous electrolyte solution contains at least one selected from the group consisting of phosphate esters, halogen-substituted phosphate esters and condensed phosphate esters. Patent Literature 4 discloses that the use of a mixed solvent of a specific halogen-substituted phosphate ester compound and a specific ester compound as an electrolyte solution solvent can provide an electrolyte solution which has a low viscosity and excellent low-temperature characteristics. Patent Literature 5 discloses a production method of a battery which uses a nonaqueous electrolyte solution added with a vinylene carbonate and 1,3-propane sultone. Patent Literature 6 discloses a battery which has a nonaqueous electrolyte solution which contains a predetermined amount of phosphate esters having a fluorine atom in the molecular chain, and in which a concentration of a salt is 1 mol/L or higher, and which viscosity is lower than 6.4 mPa·s. It is assumed that making such a constitution can provide a battery having flame retardancy, self-extinguishability and high-rate charge/discharge characteristics.
Patent Literature 7 discloses a nonaqueous electrolyte solution which contains at least one phosphate ester derivative represented by a predetermined formula, a nonaqueous solvent and a solute. Patent Literature 8 discloses that the use of a fluorophosphate ester compound for a nonaqueous electrolyte solution can provide an electrolyte solution which is excellent in conductivity and reduction resistance, and which develops high flame retardancy even in a low amount blended.
Patent Literature 9 discloses an electrolyte solution which contains a solvent containing a halogenated ethylene carbonate, a phosphate ester, and at least one phosphorus-containing compound selected from the group consisting of phosphate esters and phosphazene compounds. It disclosed that the use of the electrolyte solution can improve chemical stability in high temperatures. Patent Literature 10 discloses a nonaqueous electrolyte solution in which a lithium salt is dissolved in a nonaqueous solvent containing a phosphate ester compound, a halogen-containing cyclic carbonate ester and a chain carbonate ester. Patent Literature 11 discloses a nonaqueous electrolyte solution which contains an organic solvent containing a predetermined amount of a fluorine-containing phosphate ester represented by a predetermined formula, and an electrolyte salt. It disclosed that the electrolyte solution has the noncombustibility and flame retardancy useful for an electrolyte solution of a lithium secondary battery, has a high solubility of the electrolyte salt, has a large discharge capacity, and is excellent in charge/discharge cycle characteristics.
Patent Literature 12 describes a composition for a polymer solid electrolyte containing a fluorine-containing phosphate ester. The Patent Literature discloses a polymer crosslinking material composed of a combination of an epoxy group- and/or an oxetane ring-containing polymer, and a cationic polymerization initiator.